The otters dive eagerly into the shallow water, churning twisting trajectories that trace their frantic underwater chases. I’d read that biologists in eastern Colombia have observed giant river otters swimming with Amazon River dolphins. Indeed, the otters’ water antics appear dolphinlike, until, that is, they surface, roll over on their backs, grab fish with their webbed paws and wolf them down.

Crunch! One otter snaps the backbone of a piranha with its powerful molars while another torpedoes past my canoe to emerge with a stubby two-foot-long pintado catfish flapping in its mouth. Then, as if on cue, all this hyperactive commotion stops, and the otters launch into some impromptu landscaping around the den. They huff and sniffle, rearranging the lianas, or vines, and tear at a floating raft of water hyacinth. Then the merry troupe is off again, snorting and splashing, playing what seems like an energetic game of tag as they head down the river and around the bend.

Kallerhoff catches up to scold me about getting too close. “In Manu Biosphere Reserve in Peru, the giant otters stopped breeding because boatloads of tourists were forever invading their space,” she says. These otters didn’t appear stressed, I protest, and they continued to feed. This appears to mollify Kallerhoff. But I can see that keeping tourists safe distances from these appealing creatures will be a monumental challenge.

Other threats to the giant otters’ well-being are more insidious. The Pantanal is like an immense sponge that soaks up water from surrounding uplands and thus acts as a giant settling pond for waterborne pollution. Biologists fear that levels of mercury, for instance, may be rising.

Much of Brazil and its neighbors are still in the grips of the search for El Dorado—gold. It is largely a quest of smallscale miners, but their collecting efforts add up. “Just within the Amazon basin, some 1.2 million people extract roughly 200 tons of gold a year,” says Frank Hajek, who comanages the Frankfurt Zoological Society’s giant otter project in Peru, “and the production of each gram of gold requires one to four grams of mercury.” Up to 40 percent of this mercury escapes into the environment. An estimated 128 tons a year leaches into the Amazon alone.

“Our research in Manu, Peru, and the nearby gold mining areas shows that mercury levels in fish are too high for human consumption,” Hajek says. “At the same time, analysis of the [Peruvian] giant otters’ scat [feces] shows no traces of methylmercury and, since the otters eat primarily fish, this means that mercury must be accumulating in their bodies in toxic concentrations.” Hajek hasn’t yet been able to sample otter tissue to prove his theory. But he fears that many otters will die from mercury poisoning unless something is done. The solution, he says, is relatively simple. Miners could heat their ore in a closed vessel, capturing most of the mercury. But miners dislike this process—it produces a discolored lump of gold that fetches lower prices. Hajek says that giant river otters are also feeling the squeeze from ever diminishing rain forests. Although the home range of a typical otter family is only about 270 acres, they need thousands of acres to thrive. Young otters leaving their family often travel long distances on the water in search of the right den habitat, which usually includes a shallow lake, an abundance of fish and high banks in which to tunnel.

One biologist has estimated the Pantanal’s giant otter population at a relatively healthy 500, but there have been no measurements of mercury levels in the otters here. On my way out of Rio Negro, I take in one last view of their home from the window of the Cessna. In the course of a half hour, I spot flocks of spoonbills, egrets and storks, hundreds of caimans and capybaras, a lone swamp deer and tapirs. But the future of this abundance is far from certain. Despite a World Wildlife Fund initiative that saw UNESCO declare more than 96,000 square miles of the area a Biosphere Reserve in November 2000, only about 3 percent of the Pantanal is actually protected. The rest is in the hands of ranchers torn between development and conservation.

Recently, Conservation International of Brazil proposed creating a network of biodiversity corridors—continuous and unfenced strips of wild habitat that allow animals to range freely—throughout the Pantanal and the surrounding cerrado uplands. These corridors, carved mostly through private properties, would either be left uncultivated or farmed in an environmentally and otter-friendly manner. It sounds like a good idea: the ranchers would gain tax breaks and tourism opportunities, and the animals would get the room they need.

“The giant otters are perhaps our most captivating animals,” biologist Reinaldo Lourival, who leads the CI Pantanal branch, told me. “They can be easily glimpsed by visitors and so have become an umbrella species for conservation in the Pantanal. If we can ensure an adequate habitat for giant otters, much of our amazing biodiversity will be taken care of as well.”

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